Free radical polymerization reactions typically have an initiator system. In some applications, the initiator system is a photoinitiator system that can be based on various chemical approaches. For example, free radical polymerization reactions can be initiated using a three-component photoinitiator system that includes an electron acceptor, an electron donor, and a sensitizing compound. Alternatively, an electron donor in combination with a sensitizing compound can be used as a photoinitiator system.
In a three-component photoinitiator system that includes an electron donor, electron acceptor and a sensitizing compound, there is typically no direct reaction between the electron donor and the electron acceptor. Rather, the sensitizing compound usually absorbs actinic radiation resulting in the formation of an excited sensitizing compound. The electron donor can donate an electron to the excited sensitizing compound. That is, the sensitizing compound can reduced and the electron donor is oxidized. The reduced sensitizing compound can be a radical anion that can donate an electron to an electron acceptor to yield an initiating free radical for the polymerization reaction. The initiating free radical can be the reduced electron acceptor. In some instances of a three-component photoinitiator system, the oxidized electron donor can be a radical species that also functions as an initiating free radical.
Other photoinitiator systems have a sensitizing compound and an electron donor but no electron acceptor. The sensitizing compound can absorb actinic radiation to form an exited sensitizing compound. The electron donor can donate an electron to the excited sensitizing compound resulting in the oxidation of the electron donor. The oxidized electron donor can be a radical species that functions as an initiating free radical for polymerization reactions.